1. Field of the Invention
The present invention relates to an inner shield for cathode ray tube, and more particularly, to an improved inner shield capable of maintaining its tubular shape without distortion.
2. Description of the Related Art
A cathode ray tube (CRT) is generally used as an image displaying unit for television, computer monitors, etc. The CRT displays images by emitting visual rays when electron beams emitted from an electron gun hit a screen panel coated with red, green, and blue phosphors.
A CRT includes a screen panel for showing an image, an electron gun for emitting electron beams, a deflection yoke for deflecting a travel direction of electron beams, a shadow mask for transmitting electron beams, and a funnel holding the above elements.
The screen panel and funnel are sealed together for maintaining a vacuum inside the funnel such that electrons progress in a suitable manner therein.
The electron gun is installed inside the funnel, and the deflection yoke is circumferentially mounted around the funnel. The shadow mask is attached by welding to a mask frame, which is mounted inside the screen panel by means of springs and stud pins. The screen panel is patterned by red, green, and blue phosphors on an inner surface of the screen panel.
The elements such as the shadow mask, mask frame, and plate springs installed inside the CRT are made of ferromagnetic materials, i.e. Fe, such that the elements are magnetized to like poles according to variation of earth magnetic field, resulting in distorting the reflection of the electron beams. Accordingly, the electron beams may not hit correct phosphors.
In the conventional CRT, to solve the above described problems, an inner shield is mounted by using a clip on a rear part of the mask frame.
The inner shield is made out of a steel sheet having low permeability and in a similar shape with the funnel for preventing the reflection of the beam from being obstructed. The inner shield is either an integral type or a separable type of which two parts are assembled.
FIG. 5 is a drawing overlapping a development view with a top plane view of a prior art inner shield.
In FIG. 5, reference numeral 2 denotes workpieces cut and bent according to a predetermined design. In one workpiece 2a, a first protrusion part 4 is formed with indented portions 4a and 4b, and in the other workpiece 2b, a second protrusion part 6 is formed with a plurality of bands 6a and 6b.
The inner shield is integrated by binding the bands 6a and 6b of the second protrusion part 6 around the first protrusion part 4.
The inner shield includes opposite side walls 10 having windows 8 and flanges 12 continuously formed in a lower part of the side walls 10. A central opening 14 is formed in a longitudinal direction of an axis of the CRT such that the electron beams pass through the central opening 14. The flanges 12 are welded to the mask frame (not shown).
To assemble the inner shield precisely, widths of the indented portions 4a and 4b may be formed 2 mm wider than the width of the bands 6a and 6b such that a gap of approximately 1 mm is formed between side walls of the indented portion 4a and 4b and side edges of the bands 6a and 6b when the indented portions 4a and 4b are bound by the bands 6a and 6b. The inner shields assembled in this manner are typically stacked for subsequent process.
However, if a plurality of inner shields are stacked, the inner shields, except for the one on top of the stack, receive a load equal to the number of inner shields stacked thereon, such that gaps between the indented portions 4a and 4b and the bands 6a and 6b are distorted.
That is, a left side wall of the upper indented portion 4a and a left side edge of the upper band 6a contact each other, and a right side wall of the lower indented portion 4b and a right side edge of the lower band 6b contacted each other by concentration of the load.
Accordingly, the coupling portion of the first and second protrusions 4 and 6 droops so as to distort the contour of the inner shield. Particularly, the distortion of the central opening and the window deteriorates the ability of the inner shield to block the earth magnetic field so as to cause deterioration in purity of the CRT.
In more serious cases, the distorted inner shield obstructs the progress of the electron beams such that the electron beams do not reach the screen panel of the CRT.